When any material having a dielectric constant is present between two electric conductors, if a voltage or a current is applied to both ends of the two electric conductors, a capacitor is formed to generate capacitance which represents charge accumulation capacity, in which a magnitude of capacitance has a correlation between a dielectric constant of a dielectric substance between the two electric conductors and an opposing area and an opposing distance between the two conductors.
As commercial capacitors manufactured by the above principle, there are various kinds such as a ceramic capacitor, an electrolytic capacitor, and a Mylar capacitor.
The capacitance of the commercial capacitor generally has a size of 1 pico Farad (1 pF), 1 nano Farad (1 nF), or 1 micro Farad (1 μF), or more and may be easily measured by a meter like a multi meter.
However, in the case in which the capacitance is equal to or less than 1 pico Farad (1 Pf), when the capacitance is measured by a simple meter like the multi meter, the capacitance may not be precisely measured due to an error in the meter and/or the capacitance which may not be measured by the meter, that is, in the case of measuring a magnitude of capacitance formed between a human hand and an elevator button, there may be a case in which the capacitance may not be measured by the meter.
In this case, the capacitance may be indirectly measured. FIG. 1 illustrates an example of a method of indirectly measuring capacitance.
Referring to FIG. 1, like the case of the capacitor formed between the human hand and the elevator button, a capacitor c1 which does not know the magnitude of capacitance and is not charged is connected to point P1 of a system for detecting capacitance and there is a need to know the magnitude of the capacitance C1 which is the capacitance of the capacitor c1 using the detection system of FIG. 1. A signal detector of FIG. 1 is a detector which detects a voltage of the point P1. When the voltage of the point P1 is detected by the signal detector of FIG. 1, it is possible to know the magnitude of the capacitance C1 which is the capacitance of the capacitor c1 by an operation.
When a capacitance C2 of a capacitor c2 charged with a voltage having an amplitude of V1 is known and the capacitor c2 is connected to the point P1 by turning on a switch SW of FIG. 1, a potential Vp1 at the point P1 is determined by the following <Equation 1>.
                                          V            P                    ⁢          1                =                  V          ⁢                                          ⁢          1          ⁢                                    C              ⁢                                                          ⁢              2                                                      C                ⁢                                                                  ⁢                2                            +                              C                ⁢                                                                  ⁢                1                                                                                  <                ⁢        Equation        ⁢                                  ⁢        1        ⁢                  >                    
The capacitance C1 of the capacitor c1 which needs to be known is defined by the following <Equation 2>.
                              C          ⁢                                          ⁢          1                =                  C          ⁢                                          ⁢          2          ⁢                      (                                                            V                  ⁢                                                                          ⁢                  1                                                                      V                    P                                    ⁢                  1                                            -              1                        )                                                        <                ⁢        Equation        ⁢                                  ⁢        2        ⁢                  >                    
If it is assumed that in the above <Equation 1> detected by the signal detector of FIG. 1, the potential of Vp1 is set to be 5V, the capacitance C2 is set to be 1 pF, the V1 is set to be 10 V, it may be known from the operation of the above <Equation 2> that the magnitude of the capacitance C1 is 1 pF. Therefore, when the system for detecting capacitance is configured as illustrated in FIG. 1, it is possible to know the magnitude of capacitance without using the meter.
When the system for detecting capacitance as illustrated in the embodiment of FIG. 1 is applied to a detection system for detecting a change in distance, that is, a displacement between two objects, it is possible to detect the magnitude of capacitance and it is possible to detect the displacement using the detected capacitance.
FIG. 2 illustrates an embodiment of a sensor for detecting a displacement, in which two pistons are made of plastic or glass or face each other at a predetermined distance inside a virtual cylinder. Referring to FIG. 2, the two pistons having the same area marked by “S” face each other at a first distance “d1” inside the cylinder. When a force is applied to an upper piston to make an upper piston approach a lower piston and a distance between the upper and lower pistons is changed to “d2”, “d1-d2” which is a moving distance of the upper piston may be known by using the detection system illustrated in FIG. 1.
Referring to FIG. 2, a material of which the permittivity is “∈1” is filled between the pistons of FIG. 2 and in the case in which the opposing area is “S” and the opposing distance is “d”, a capacitance CVR of a capacitor c4 which is formed at both ends of the piston is defined by the following Equation 3.
                    CVR        =                  ɛ          ⁢                                          ⁢          1          ⁢                      S            d                                                        <                ⁢        Equation        ⁢                                  ⁢        3        ⁢                  >                    
If the known first distance “d1”, the known opposing area “S”, and the known permittivity “∈1” are substituted into the above <Equation 3>, when the first distance is “d1”, “CVR1” which is the magnitude of capacitance formed between the two pistons of FIG. 2 may be known. Further, “CVR2” formed by any “d2” may be extracted based on the system for detecting capacitance of FIG. 1 and the operation of the above <Equation 1> and <Equation 2> and when the extracted “CRV2” is substituted into the above <Equation 3>, the “d2” may be known and therefore the moving distance of the upper piston of FIG. 2 may be known by the operation of “d1-d2”.
FIG. 3 is a diagram illustrating an embodiment of a system for detecting capacitance in which a system for detecting capacitance illustrated in FIG. 1 is more embodied. Referring to FIG. 3, one side of a capacitor c4 generated by the configuration of FIG. 2 is connected to point P2 of FIG. 3 and the other side of the capacitor c4 is connected to a ground of the system configuring FIG. 3. c2 and c3 are a parasitic capacitor of the detection system of FIG. 3. For example, c2 may be an element formed by a layout in which the signal detector of FIG. 3 is embedded in an IC and a wiring of the point P2 reaching a switching device SW1 and the signal detector is wired with any signal line at a minute interval, intersects the signal line, or the like and the parasitic capacitor c3 may also be an element formed by modeling the parasitic capacitor formed between a gate which is an input unit of a circuit element configuring the signal detector of FIG. 3 and a system ground. The parasitic capacitor may not be limited and is variously distributed according to the configuration of the detection system.
When the switch SW1 is turned on in a state in which a switch SW2 of FIG. 3 is turned off and thus a charging voltage marked by “Vchg” is supplied to the point P2, capacitors connected to the point P2, that is, c2/c3/c4 are charged with the “Vchg” and when the capacitor c1 is in a disconnection state from the point P2 due to the turn off of the switch “SW2”, the capacitor c1 is not affected by the “Vchg” (small letters c2/c3/c4 are a capacitor and capital letters C1/C2/C3/C4 are a capacitance of each capacitor). Next, when the switch “SW2” of FIG. 3 is turned on and thus the capacitor c1 charged with “V2” is connected to the point “P2”, a potential “Vp2” which is detected by the signal detector of FIG. 3 is defined by the following <Equation 4>.
                                          V            P                    ⁢          2                =                              V            chg                    +                      V            ⁢                                                  ⁢            2            ⁢                                          C                ⁢                                                                  ⁢                1                                                              C                  ⁢                                                                          ⁢                  1                                +                                  C                  ⁢                                                                          ⁢                  2                                +                                  C                  ⁢                                                                          ⁢                  3                                +                CVR                                                                                  <                ⁢        Equation        ⁢                                  ⁢        4        ⁢                  >                    
In the above <Equation 4>, the Vp2 is detected by the signal detector of FIG. 3 and thus an amplitude of Vp2 may be known and when all the amplitudes of the V2, Vchg, and C1/C2/C3 are known, the magnitude of CVR may be extracted by the operation. Further, it is possible to know a size of the distance “d2” by substituting the CVR into the above <Equation 3>. Therefore, when the first distance “d1” is known in advance, the “d1-d2” which is a displacement of the upper piston may be known.
However, the system having the modeling has several problems in detecting the CVR variation depending on the displacement of the upper piston of FIG. 2.
For example, if it is assumed that in the above <Equation 4>, C1=C2=C3=10 pF, CVR=1 pF, Vchg=1V, and V2=10V, Vp2=4.2258 V. When the CVR is changed by 0.1 pF from 1 pF to 0.9 pF due to the displacement of the upper piston, the Vp2 detected by the signal detector of FIG. 3 is 4.2362 V, and therefore when the CVR is changed by 0.1 pF, the variation of the Vp2 is only 4.2362−4.2258=0.0104V, that is, 10.4 mV. Although not illustrated in the signal detector of FIG. 3, an ADC which converts an analog value which is a voltage detected by the signal detector into a digital value has been used. The detected Vp2 approximates 4.2 V, and therefore the ADC of the signal detector for detecting the amplitude of Vp2 sets about 2.5V to 4.5V as a detection band. If it is assumed that a 10 bit ADC is used in a detector, the 10 bit ADC needs to detect 4.5V-3.5 V, that is, 1 V, and therefore resolution of the ADC is “1V/1024 bits” and resolution per one bit is around 1 mV. As a result, when a signal variation of a displacement sensor of FIG. 2 is 10.4 mV, only 1% of performance of the 10 bit ADC having the resolution of 1024 bits may be used.
Generally, a system having a signal to noise ratio (SNR) of 1% corresponds to a good system, but noise generally exceeds several % of a signal. Therefore, if a size of the detected signal is only around 1%, when the noise is several %, it is difficult to distinguish the signal from the noise and therefore the detected signal has reduced reliability.
As a result, when a change in magnitude of capacitance CVR detected by the system, there is a problem in that the detected signal may not be reliable.